This invention relates to a metal-core printed wiring board, particularly one having a high heat-dissipative capacity, and to a process for manufacturing it. The development of small and sophisticated electronic parts and of high-density packaging techniques therefor have resulted in a strong demand for such printed wiring boards.
A metal-core printed wiring board has a wiring pattern formed on a metal core base which is coated with an insulating material. A base with a metal core has a number of advantages over a base formed of other material such as glass or an epoxy resin. These advantages include higher heat-dissapating capacity, higher mechanical strength, better electromagnetic sheilding effect and, lower manufacturing cost. Thus, there are known a variety of types of metal-core printed wiring boards and a variety of processes for manufacturing them.
One of these processes is disclosed in Japanese Laid-Open Patent Specification No. 74091/1983, in which a sheet of aluminum having a through hole is electrically insulated by Alumite treatment (anodic oxidation), and a wiring pattern is formed thereon by copper plating. This process has a number of drawbacks. First, only a sheet of aluminum may be used as the metal core. Further, the insulated portion of the board can withstand only a low voltage, and does not permit the mounting thereon of any component part of the kind to which a high voltage is applied. Third, both the Alumite treatment and the formation of a wiring pattern by copper plating take a long time.
Another known process is disclosed in Japanese Laid-Open Patent Specification No. 132989/1983, in which a metal sheet having a through hole, a prepreg sheet composed of a fabric of glass and an epoxy resin, and a copper foil are pressed together, whereby the prepreg sheet forms an insulating layer and a part of the resin flows from the prepreg sheet to fill the through hole, while the copper foil is simultaneously bonded to the prepreg. A circuit pattern then is formed on the base by a known subtractive process. However, the epoxy resin filling the through hole has a coefficient of linear expansion which differs substantially from that of the metal forming the core of the base. As a result, the resin is likely to separate easily from the through hole when a solder heat resistance test or a heat cycle test is conducted on the base. Moreover, as the glass and epoxy resin forming the insulating layer have a low thermal conductivity, the base as a whole also has a low thermal conductivity, and so cannot dissipate heat satisfactorily.
In other known processes, an insulating layer if formed by the electrodeposition of an insulating material on a core, or the fluidized bed coating of a core with a powdery paint. However, these processes still have various problems, as discussed above. A circuit pattern may be difficult to form; the insulating material may have a coefficient of thermal expansion which is substantially different from that of the metal forming the core; or the insulating layer may have a low thermal conductivity.